The present invention outlines a method and apparatus to determine the state of charge (SOC) of a lead-acid battery operating in a hybrid electric vehicle (HEV).
In today""s automotive market, a variety of propulsion or drive technologies can be used to power vehicles. The technologies include internal combustion engines (ICEs), electric drive systems utilizing batteries and/or fuel cells as an energy source, and hybrid systems utilizing a combination of internal combustion engines and electric drive systems. Each propulsion system has specific technological, financial, and performance advantages and disadvantages, depending on the state of energy prices, energy infrastructure developments, environmental laws, and government incentives.
The increasing demand to improve fuel economy and reduce emissions in present vehicles has led to the development of advanced hybrid vehicles. Hybrid electric vehicles (HEV) are classified as vehicles having at least two separate power sources, typically an internal combustion engine and an electric traction motor. A hybrid electric vehicle will generally operate with a high-voltage battery pack (xe2x89xa742 V) operating an electric motor running in conjunction with the ICE.
Hybrid vehicles, as compared to standard vehicles driven by an ICE, have improved fuel economy and consequently reduced emissions. During varying driving conditions hybrid vehicles will alternate between separate power sources, depending on the most efficient manner of operation of each power source. For example, a hybrid vehicle equipped with an ICE and an electric motor will shut down the ICE during a stopped or idle condition, allowing the electric motor to restart the ICE and eventually propel the vehicle, improving fuel economy for the hybrid vehicle.
Hybrid vehicles are broadly classified into series or parallel drive trains, depending upon the configuration of the drive trains. In a series drive train utilizing an ICE and an electric traction motor, only the electric motor drives the wheels of a vehicle. The ICE converts a fuel source to mechanical energy to turn a generator, which converts the mechanical energy to electrical energy to drive the electric motor. In a parallel hybrid drive train system, two power sources such as an ICE and an electric traction motor operate in parallel to propel a vehicle. Generally, a hybrid vehicle having a parallel drive train combines the power and range advantages of a conventional ICE with the efficiency and electrical regeneration capability of an electric motor to increase fuel economy and lower emissions, as compared with a traditional ICE vehicle.
Battery packs having secondary/rechargeable batteries are an important component of hybrid vehicle systems, as they enable an electric motor/generator (MoGen) to store braking energy in the battery pack during regeneration and charging by the ICE. The MoGen utilizes the stored energy in the battery pack to propel or drive the vehicle when the ICE is not operating. During operation, the ICE will be shut on and off intermittently, according to driving conditions, causing the battery pack to be constantly charged and discharged by the MoGen. The state of charge (SOC, defined as the percentage of the full capacity of a battery that is still available for further discharge) is used to regulate the charging and discharging of the battery.
Currently, the most cost-effective, commercially-ready battery for HEV applications is the lead-acid battery. Lead-acid batteries have been widely used in the automotive industry for starting-lighting-ignition applications in the last hundred years. However, in a hybrid application, the power loads and usage are much heavier than that used in previous lead-acid battery applications. To operate efficiently in HEV applications, a lead-acid battery needs to operate near its optimal SOC to maximize its discharge and charge power capabilities.
The discharge and charge reactions in the lead-acid battery are not symmetric, as in other battery technologies such as nickel-metal hydride and lithium-ion batteries. That is, the discharge battery resistance is typically lower than the charge battery resistance, which includes secondary gassing reactions at a SOC greater than 60%. Consequently, it is difficult to predict the SOC based on charge voltages. Accordingly, a voltage-based SOC for a lead-acid battery is usually based on discharge data only. However, in a HEV application where there are continual regeneration events, the battery discharge voltages are shifted higher due to increased concentration of sulfuric acid after a charge. The present invention provides a method and apparatus to accommodate the effect of regeneration on the discharge voltage, and thus predict a more accurate and consistent SOC.
The present invention integrates three independent methods for determining the battery SOC of an electric or HEV during vehicle operation. The three independent methods include: a current-based SOC (ISOC) based on amp-hour (Ah) integration; a voltage-based SOC (VSOC) based on a calculated open circuit voltage (OCV); and a rest-based SOC (RSOC) based on a measured OCV after the vehicle has been powered-off. The present invention is optimized to determine the SOC of a lead-acid battery using factors uniquely relevant to a lead-acid battery. For example, the present ISOC method accommodates for charge inefficiency due to secondary gas reactions and current (Peukert) effect on the discharge capacity. The VSOC and RSOC calculations in the present invention consider the discharge data in predicting the OCV at any time during operation of a battery and compensate for regeneration effects.
The SOC determination of the present invention relies heavily on the VSOC and is adaptive to changes in the battery due to temperature and aging. The method of the present invention will periodically reset the ISOC to match the VSOC because of possible errors in amp-hour integration from small inaccuracies in current measurements and/or charge inefficiency of the battery over time. Furthermore, when the vehicle is off, a measured open circuit voltage can be correlated to the RSOC. The difference in the RSOC and the VSOC provides the basis for adaptation of battery pack capacity due to battery degradation resulting from hybrid cycling at partial SOCs and/or aging.